New Research: First Genome Therapy to treat Hemophilia

Hemophilia is one of the oldest genetic diseases known, famous for affecting many descendants of Queen Victoria. Back in the Babylonian times, it was described in Talmud texts that "if circumcised her first child and he died, and a second one also died, she must not circumcise her third child." Hemophilia is a genetic clotting disorder, which occurs when the gene for one of the many clotting factors is mutated. During blood-clotting, many proteins get activated which ultimately lead to formation of a net made of fibrin, trapping blood cells and forming the blood clot. In hemophilia, fibrin is no longer created so even a small bruise is extremely deadly.

These days, hemophilia is a chronic disease, treated with injections of factor into the bloodstream every other day to restore the missing protein. This is still not an optimal solution since infections can be an issue as well as possible contamination of the factor supply with disease agents (ex. AIDS). In addition, factor is extremely cost-prohibitive, costing upwards of $300,000 a year.

Since it is well known that these diseases are caused by genetic defects, adding back a functional gene should in principle cure hemophilia, the basic premise of gene therapy. However, one problem with gene therapy is how do you fix the defect. In one famous trial ten years ago, scientists added back a functional gamma-c gene to treat children with SCID-X, a disease where the immune system is so defective that children have to live in a bubble. Several of the patients began to produce functional immune system cells, curing them of SCID-X. However, several years later, two patients developed leukemia due to the yC gene being randomly inserted near the cancer-causing LMO2 gene. As a result, one goal of gene therapy today is to precisely insert the gene into the appropriate location and prevent non-random integrations.

Hojun Li from the High lab at the Children's Hospital of Philadelphia described a potential cure for hemophilia B, caused by a mutation in Factor 9 (F9). In it, they use the technique of zinc-finger nucleases developed in California by Sangamo. Zinc-fingers are a structure in proteins that recognize specific sequences in DNA. If one strung together a bunch of zinc-fingers, one can make it bind to a specific spot in the genome. Now if we add on an enzyme that cuts DNA called a nuclease, one creates a nick at a very specific spot in the genome. If one then adds in a copy of the functional gene, it can be swapped into the genome through a process called homologous recombination.

Li took mice engineered to have a F9 mutation and infected them with two viruses, one containing the zinc-finger nuclease and another containing the functional copy of the gene. The specific virus strain they used (hepatotropic adeno-associated virus) specifically infects liver cells, the site where clotting factors are made in people. After a few weeks post-treatment, they discovered that the liver was now producing functional F9 and the mice could now clot blood. More importantly, they found the functional gene copy inserted precisely into site of interest and no-where else. Not only were these mice “cured” of hemophilia, the dangers discovered with the SCID-X trial indicated above should not occur in these mice.

This is a promising study. One can foresee a quick movement towards clinical trials and treatments in humans, especially because these mice engineered to use human variants of F9. If this method is found to be safe, zinc-finger nucleases may become the norm for treating many genetic diseases. Indeed, one could say that hemophilia may become the first genetic disease officially cured.



Li H, et al. (2011) In vivo genome editing restores haemostasis in a mouse model of haemophilia. Nature, [Epub ahead of print Paper.

Cavazzana-Calvo, M., et al. (2000) Gene therapy of human severe combined immunodeficiency (SCID)-X1 disease. Science, 288(5466):669-72. Paper